Magnetic structures for traveling wave tubes



June 21, 1960 c. c. CUTLER MAGNETIC STRUCTURES FOR TRAVELING WAVE TUBESFiled June 6. 1957 2 Sheets-Sheet 2 INVENTOR C. C. CUTLER BY ATTORNEYMAGNETIC STRUQTURES FOR TRAVELING WAVE TUBES Cassius C. Cutler,Gillette, N.J., assignor to Bell Telephone Laboratories, Incorporated,New York, N311, a corporation of New York Filed June 6, 1957, Ser. No.664,015

10 Claims. (Cl. 315--3.5)

This invention relates to traveling wave tubes and more particularly tosuch tubes wherein an electron beam is focused by a magnetic field alonga relatively long path. This application is a continuation-in-part ofapplication Serial No. 168,202, filed June 15, 1950.

In certain electron discharge devices, such as traveling wave tubes, anelectron stream is projected into an interaction'space generally definedby a helix, where it is made to interact with an electromagnetic wavetraveling along the helix. Optimum operation is achieved when theelectron stream is confined to a substantially cylindrical form havingelectrons at its radial extremities close to but not impinging the helixthroughout the interaction space. It has been the practice to establisha longitudinal .magnetic field along the path of electron flow tominimize transverse components of the electron stream caused chiefly byspace charge effects and thus to confine the beam as desired.

A uniform longitudinal magnetic field of sufficient strength to oflisetthe large space charge forces existing .in an electron stream of highdensity over a relatively long electron path may be provided by a groupof long parallel permanent magnets positioned about the interactionspace as disclosed in the parent application, or by a single aperturedpermanent magnet as disclosed in M. S. Glass Patent 2,791,718, issuedMay 7, 1957, and in P. P. Ciofii Patent 2,871,395, issued January 27,1959. One ditficulty encountered in the use of .such structures is thatavailable permanent magnet materials are not sufficiently .uniform inmechanical structure to provide entirely regular external fields.

The resultant fields contain transverse components which may adverselyaffect the electron path through the interaction space.

It is an object of this invention to improve the magnetic focusing ofelectron streams in electron discharge devices such as traveling wavetubes.

More specifically it is an object of this invention to provide amagnetic field which will focus an electron stream over a relativelylong path, confining the stream to a uniform shape within narrow limits.

Another object of this invention is to assure a precisely straightmagnetic field by removing any transverse comverse plates of a magneticmaterial are used to eliminate transverse irregularities in the magneticfield.

Specifically, in accordance with this invention, a series ofspaced-apart apertured magnetic members are posit'ioned about theinteraction space in a traveling wave States Patent tube. The desiredstraight magnetic field is directed along the axis of the tube throughthe apertures in the magnetic members. The magnetic members arecarefully aligned in parallel planes perpendicular to the field throughthe apertures so that transverse components of the field may beshort-circuited.

The magnetic members are of high permeability material to provide a lowreluctance path for such transverse field components, but are extremelythin so as not to exert an influence on the longitudinal fieldcomponents. In this fashion the longitudinal field is not influenced bythe magnetic members to divert its path and disrupt the strong straightfield desired. However, the undesirable perpendicular component oftransverse fields due primarily to imperfections inthe field producingmagnetic material, are removed from the interaction space by themagnetic members without disturbing the desired longitudinal componentsof such transverse fields.

By proper spacing and shaping of the thin magnetic discs, concentrationof the longitudinal magnetic field in particular areas for optimumresults with specific field producing magnet designs may be realized.

It is a feature of this invention that traveling wave tube apparatuscomprise a plurality of spaced-apart high permeability magnetic membersto shunt transverse components of a magnetic field passing throughapertures in said members.

It is another feature of this invention that the magnetic members hemade thin enough to affect only transverse components of the magneticfield.

it is a further feature of this invention that the magnetic members bepositioned in parallel planes perpendicular to the longitudinal magneticfield.

It is a feature of one specific embodiment of this invention that themagnetic members be shaped at their extremities for increased fielduniformity.

It is another feature of one specific embodiment of this invention thatthe spacing between magnetic members be variedto concentrate the fieldin desired locations.

A complete understanding of this invention and of the various featuresthereof may be gained from consideration of the following detaileddescription and the accompanying drawing in which:

Fig. 1 is a perspective view of a magnetic field straightener assemblyutilized in one specific embodiment of this invention;

Fig. 2 is a side view in section of traveling wave tube apparatusutilized in one specific illustrative embodiment of this invention,employing the field straightener assembly of Fig. 1;

Fig. 3 demonstrates the effect of the field straightener assembly ofFig. 1 on the magnetic field; and

Fig. '4 shows two means for shaping the magnetic members in accordancewith another specific embodiment of this invention.

Referring particularly to Fig. 1, the magnetic field straightenerassembly shown generally as 1 comprises a series of thin discs 2 of highpermeability material spaced apart by discs 3 of nonmagnetic materialsuch as aluminum. The discs are apertured and mounted to permitalignment of the apertures, for example on a hollow mandrel 4, also ofnonmagnetic material. Supports 5, adjacent the ends of the assembly,permit mounting of the assembly in the magnetic circuit of the travelingwave tube and adjustment relative, to the axis of the assembly to assureprecise perpendicular relationship between the parallel planes formed bythe magnetic discs 2 and the magnetic field through the apertures.

The mounting is illustrated more clearly in Fig. 2 wherein a permanentmagnet assembly 10 has a central aperture 11 along the axis of symmetrytherethrough. The

magnet assembly forms a strong straight magnetic field along the axis ofsymmetry suitable for focusing an electron stream and may comprise fourlong permanent magnet bars. Other types of magnet structures known inthe art such as disclosed in the priorly cited Glass and Ciofli patents,are suitable for this purpose, the instant magnet assembly 10 merelyserving as an illustration of the proper environment for this invention.The traveling Wave tube 12, which'also may be of any type known in theart, is inserted into the aperture '11 between magnetic members 1d andextended through the field straightener assembly 1. The traveling wavetube essentially comprises an electron gun assembly 13 a helixtransmission circuit 14 and an electron collector assembly 15. Waveguide input and output coupling means 16 are shown. The rods 17, seatedin supports 5, assist in positioning of the field straightener assembly1 so as to assure that the planes including the discs 2 are preciselyperpendicular to the longitudinal magnetic lines of force extendingalong the helix portion of the tube 12 through the field straightenerassembly 1.

The basic requirement of straight field focusing in contradistinction toperiodic focusing as known in the art, is the provision of a strongstraight magnetic field over a relatively long air space constitutingthe interaction space of a traveling wave tube. Careful design of thefieldproducing magnet permits provision of a substantially straightfield. Nevertheless, imperfections of microscopic proportion in themagnetic material may create transverse fields which disrupt the desiredoperation. The magnetic discs 2 serve to attract and short-circuit theundesirable perpendicular components of these transverse fields. Theyare made extremely thin to assure that only the perpendicular fieldcomponents are attracted by the low reluctance path they provide, whileleaving the desirable longitudinal components of the transverse fieldsundisturbed.

The principle involved is illustrated in Fig. 3. It is known thatprovision of annular rings of magnetic material close to the path of thedesired magnetic field results in field deviations as shown in Fig. 3a.The magnetic lines of force seek the path of least reluctance, and highpermeability elements 32 and 33 provide a more attractive path than theair space therebetween. Thus longitudinal lines of force 30 and 31,which would otherwise maintain a straight path, are deviatedsubstantially in the manner shown, thereby considerably weakening thefield in the region between the magnets. This arrangement may beemployed in various tube applications to obtain a periodically variedconcentration of field along the electron beam path. It is apparent thatthe straight field along the interaction space, as desired in theinstant application, is not achieved and the use of such rings merelyaccentuates field distortion.

In accordance with this invention, in order to assure an undisturbedlongitudinal field and to remove elements of fields directed otherwisethan in the precise desired direction, extremely thin discs of magneticmaterial closely spaced in parallel planes perpendicular to the desiredmagnetic field and apertured to receive the desired field, are utilized.

Transverse or stray fields comprising lines of force, such as 34 shownin exaggerated form for illustration in Fig. 3b, may exist in anotherwise straight field directed toward the apertures of thin magneticdiscs such as 36. The transverse field line comprises components 37 and38 perpendicular and parallel respectively to the direction of desiredfield. The magnetic discs such as 36, due to their extreme thinness, donot afford a path sufficiently lower in reluctance than the air space inthe apertures to cause longitudinal magnetic lines of force through theapertures to deviate from their straight paths so as to passtherethrough. Thus the horizontal component 38 of line of force '34 willbehave as though the magnetic discs were not present. The perpendicularcomponent 37, however,

attracted by the high permeability discs.3.6 which afliord '4 r a lowreluctance path in its plane. The path of the perpendicular component 37is substantially as shown in Fig. 30; Le, completely bypassing theaperture area and preventing its exercising any influence therein. Thus,in effect, the perpendicular components are short-circuited, and theirharmful effect on the straight magnetic field is removed. The result isselective attraction of undesirable field components rather thansubstantially complete attraction of the field as achieved by theannular rings in Fig. 3a.

Magnetic disc configurations such as shown in Fig. 4a and Fig. 4b may beemployed in specific applications of the field straightener assembly.The bent or cupped extremities of the discs serve to shunt portions ofthe longitudinal field, and the shunting effect may be varied by spacingthe discs closer together adjacent the ends of the field producingmagnet structure than adjacent the central portion thereof. This methodshunting has the unique property that, while decreasing the field nearthe ends of the structure, it increases the field in the centralportion. The property of eliminating undesirable transverse fieldcomponents is retained and implemented primarily by the fiat, innerportions of the discs perpendicular to the desired longitudinal field.

t is to be understood that theabove-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departingfrom the spirit and scope of the invention.

What is claimed is: p

1. In an electron discharge device, means for pro ducing a magneticfield along a longitudinal path, means for directing an electron streamalong said path 'in said field, and a plurality of magnetic membersaligned along said path with the largest dimension in planesperpendicular to said magnetic field along said path and positionedcloser to said path than said magnetic field producing means so as toattract only transverse components of said field in said path,successive ones of said magnetic members being spaced-apart by an amountgreater than the dimension of each of said members along said path.

2. Traveling wave tube apparatus comprising an elongated helicalconductor, means for projecting a stream of electrons lengthwise of andin coupled relationship to said conductor, means for applying alongitudinal magnetic 'field along said conductor to focus said electronstream, and a plurality of coaxially aligned plates of high permeabilityapertured to receive said helical conductor and arranged in parallelplanes transverse to the direction of said magnetic field along saidconductor, successive ones of said plates being spaced-apart by anamount greater than the dimension of said plates transverse to saidplanes, the dimensions of said plates in said planes being greater thanthe dimension of said plates transverse to said planes.

3. Electron beam focusing apparatus comprising means for applying alongitudinal magnetic field along the electron beam path and a pluralityof spaced-apart magnetic members apertured to receive the electron beamand aligned substantially in parallel planes transverse to said magneticfield along the electron beam path, and between said path and saidmagnetic field applying means, the dimension of said members in thedirection of said field being less than both the spacing betweenadjacent of said members and the dimensions of said members'in saidparallel planes transverse to said magnetic field along the electronbeam path so as to provide a short circuit path for transversecomponents of said magnetic field through said apertures.

4. Electron beam focusing apparatus in accordance with claim 3 whereinsaid magnetic members are annular plates of high'permeability material.

5. Electron beam focusing apparatus in accordance with claim 4 whereinsaid magnetic members mmprise Gt. 1. l

a flat central portion and end portions angularly displaced from theplane of said central portion.

6. Electron beam focusing apparatus in accordance with claim 4 whereineach of said magnetic members is bent in the peripheral regionsubstantially to a frustoconical contour.

7. Electron beam focusing apparatus in accordance with claim 4 andfurther comprising low permeability solid members interjacent said highpermeability magnetic members.

8.. Electron discharge device apparatus comprising means for forming amagnetic field, means for projecting a stream of electrons through saidmagnetic field and a plurality of apertured spaced-apart discs ofmagnetic material aligned substantially in parallel planes surroundingsaid magnetic field and said electron stream, said discs being extremelythin so as not to attract components of said magnetic field directedperpendicular to said parallel planes.

9. Traveling wave tube apparatus comprising a plurality of aperturedmagnetic members and an elongated helical conductor extending in theapertures of said magnetic members, means for projecting a stream ofelectrons lengthwise of and in coupled relationship to said conductor,and permanent magnet means applying a longitudinal magnetic field alongsaid conductor to focus said electron stream, said magnetic membersbeing suspended between said permanent magnet means and said conductorin spaced-apart relation substantially in parallel planes mutuallyperpendicular .to said magnetic field through said apertures andproportioned such that the amount of magnetic material in said membersadjacent the apertures is sulficient to cause deviation only ofcomponents of said field which are initially directed other thanperpendicular to said parallel planes, at least one of said plurality ofmagnetic members comprising extreme portions angularly displaced fromthe plane of the balance of said one member.

10. A traveling Wave tube comprising means defining an interactioncircuit, means for projecting a stream of electrons along saidinteraction circuit, magnetic means for providing a straight magneticfield for focusing said electron stream in said interaction circuit, andmeans for removing transverse components of said magnetic field adjacentsaid interaction circuit, said last-mentioned means comprising aplurality of thin magnetic plates of high permeability positioned insaid magnetic field along said interaction circuit and perpendicular tothe direction of said straight magnetic focusing field, successive onesof said plates being spaced-apart by an amount greater than thethickness of said plates.

References Cited in the file of this patent UNITED STATES PATENTS2,300,052 Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 19422,259,531 Miller et al Oct. 21, 1951 2,636,948 Pierce Apr. 28, 19532,741,718 Wang Apr. 10, 1956 2,797,360 Rogers June 25, 1957 FOREIGNPATENTS 1,080,230 France May 26, 1954

